The present invention relates to an optical receiver for converting an optical signal into an electrical signal by means of a light-receiving element and, more particularly, to an optical receiver for optical communication used to convert a high-speed optical signal into an electrical signal and observe the waveform of the signal.
Recently, optical communication systems for transmitting information by using light have been widely used.
FIG. 4 shows an optical receiver for use in conventional optical communication systems. In this optical receiver, a DC bias terminal 13 is connected to the cathode electrode of a photodiode 11 via a resistor 12, and a bypass capacitor 14 is connected between the circuit and ground. The anode electrode of the photodiode 11 is grounded via a resistor 15. The anode electrode is also connected to an output terminal 16 to which a device such as an oscilloscope for waveform measurement is connected through a coaxial cable. In this optical receiver, an input optical signal is converted into a current Ip by the photodiode 11, and a voltage determined by a product of the resistance (normally 50 .OMEGA.) of the resistor 15 and the current Ip appears across the two terminals of the resistor 15.
In the above-mentioned conventional optical receiver, the resistance of the resistor 15 is normally set at 50 .OMEGA. so as to be matched with the impedance of a circuit or a coaxial cable to be connected to the output terminal 16. However, transient phenomena occur due to the junction capacitance of the photodiode 11 and the inductance of wiring, and this results in ringing in an output pulse waveform. Therefore, it is impossible to perform accurate waveform observation. FIG. 5 graphically illustrates an output waveform as a function of a rectangular-wave input, i.e., a signal voltage waveform at the output terminal 16. This signal voltage waveform produces ringing A in the transient state.
In addition, since the value of the resistor 15 cannot be increased in the conventional optical receiver, the voltage which appears at the output terminal 16 is also low.